Known machines for producing helically-wound lock-seam tubing are disclosed by, for example, British Patent Publication GB 2,213,748 and U.S. Pat. No. 4,706,481. These machines are well-known to the person skilled in the art and thus need not to be described in any detail here. Several further developments of such machines are available on the market, but these machines still suffer from certain drawbacks, as will be discussed in the following.
The metal strip used for the production in question is normally coated with zinc in order to be able to withstand corrosion. The metal strip is thus galvanised, which leads to problems that are well-known to the skilled person.
When the metal strip is edge-formed in the roller assembly preceding the forming head, small fragments of zinc are released from the metal strip and adhere to the edge-forming rollers. Owing to these zinc fragments, the diameter of the edge-forming rollers increases, especially in the last pair of rollers. Since these rollers are not resiliently journalled, the clamping force in the nip beneath two rollers is considerably increased as a result of the build-up of zinc on the rollers. In severe cases, these undesirable forces are so significant that the rollers are broken, which causes stoppages of production.
Another problem is that zinc fragments are released as a result of the sliding of the metal strip between the guiding plates before the forming head, which leads to a build-up of zinc on the opposite faces of these guiding plates. If the build-up of zinc on the guiding plates is too considerable, the feeding of the metal strip between the guiding plates can be disturbed.
Another disadvantage is that the zinc coating on the metal strip is softened and adheres to the insider of the forming head owing to significant frictional forces and heat development in the forming head. This renders more difficult the forming of the tubing in the forming head.
Hitherto, the problems related to the released zinc fragments have been solved by supplying lubricant, normally oil, to the metal strip on different places in the machine. For example, such lubrication is discussed in U.S. Pat. No. 2,136,943. Lubrication of metal strips is also discussed in U.S. Pat. No. 3,750,439 and the Russian Publication No. 835,558.
In more machines available on today's market, the oil is, however, supplied to the metal strip in a very simple manner. Traditionally, the lower roller of the first pair of rollers in the edge-forming assembly is permanently immersed in oil in a trough, by means of which oil is supplied over the total width of the metal strip. However, this arrangement suffers from several drawbacks, as will be discussed below.
As the roller immersed in oil in the oil trough rotates, oil is spread to the upper surface of the metal strip, which later will form the inside of the tube. The finished tubes are thus covered with oil on both the inside and the outside, which has unfavourable consequences.
If the tubes are used in ventilation duct systems, dust and other particles accumulate on the inside of the tube, which in turn becomes the breeding-ground of bacteria, virus, mould and the like, which may be dangerous to people's health. Further, accumulated dirt on the inside of the tube causes reduction of the tube inner cross-section with the consequence that the air flow rates and ventilation efficiencies for which the tube was originally designed are no longer upheld.
Similarly, dust and other particles accumulate on the outside of the tubes, making them unpleasant to handle for the personnel. Further, the tubes have to be cleaned on the outside if they are to be painted.
During transportation, tubes of smaller diameter are often inserted in tubes of greater diameter in order to save space. Oil on the outside of the smaller tubes is then transferred to the inside of the bigger tubes.
A special disadvantage is that an excess of oil in the lock seem may cause the tube to "spin" or slide during transportation and handling. As a result of such a spin, the tube diameter increases and consequently the sealing is impaired and standard components cannot be fitted on the tube.
In recent years, the demand for oil-free, helically-wound lock-seam tubing has increased, and the finding of a solution to the above-mentioned problems is therefore of great interest.